Highly active antiretroviral therapy (HAART) has dramatically increased life expectancy in HIV infection and by 2015 more than half of HIV patients in the U.S. will be older than 50. Elderly HIV patients often experience comorbidities such as neuropsychiatric complications and heart, kidney and liver disease, which require additional medications. For those patients failing HAART due to HIV drug resistance, the recently developed antiretroviral class of CCR5 antagonists represents an alternative treatment option. Maraviroc is currently approved for patients infected with drug-resistant HIV viruses that use CCR5 as a coreceptor (the so-called R5 strains) and Vicriviroc is in Phase III clinical trials. Importantly, CCR5 antagonists, unlike HIV non-nucleoside reverse transcriptase and protease inhibitors, neither induce nor inhibit cytochrome P450 3A4, which is responsible for the metabolism of many medications commonly used by the elderly. Thus, CCR5 antagonists may provide safer treatment options for elderly patients with drug-resistant R5 HIV infection because of the reduced potential for drug interaction with coadministered medications. However, recent clinical trials demonstrate that R5 HIV can develop resistance to CCR5 antagonists. Viruses resistant to CCR5 antagonists often gain the ability to use antagonist-bound CCR5, which is manifested by incomplete viral inhibition (plateau at <100%) in drug susceptibility assays. In Preliminary Results we demonstrate that these inhibition plateaus are affected by CCR5 density, with increased viral suppression achieved at lower CCR5 densities. These results provide the first indication that decreasing CCR5 can control viral resistance to CCR5 antagonists. Our long-term goal is to control HIV drug-resistance by targeting cellular components required in the HIV life cycle. The objective of this proposal is to determine the mechanism by which R5 HIV resistant to CCR5 antagonist regains sensitivity at reduced CCR5 density. Our hypothesis is that R5 HIV resistant to CCR5 antagonist regains sensitivity at reduced CCR5 density because the affinity of the resistant virus envelope protein is lower for antagonist-bound CCR5 than for free CCR5. We have one Specific Aim: To determine the affinity of CCR5 antagonist-resistant gp120 for free and bound-CCR5, the length of the lag phase for association of CD4 and CCR5 (free and antagonist-bound) and the stoichiometry (number of antagonist-bound CCR5 molecules engaged per virion) in CCR5 antagonist-resistant R5 HIV infection. We will determine the affinity of resistant gp120 for free and bound-CCR5 by binding assays, the length of the lag phase for association of CD4 and CCR5 (free and antagonist-bound) in pseudovirus infections and cell-cell fusion assays, and the stoichiometry in pseudovirus infection of cell lines with varying CCR5 densities. Successful proof of this hypothesis will suggest CCR5 internalization (i.e., using RANTES analogs) and inhibition of expression (i.e., using RNA interference) as potential therapeutic strategies to restore sensitivity to CCR5 antagonists and thus improve quality of live for HIV patients, especially for the elderly. PUBLIC HEALTH RELEVANCE: According to the 2007 Workshop on HIV Infection and Aging, organized among others by the NIAID and the NIA, "identification of particular antiretroviral drugs or treatment strategies that may be more effective in older HIV-infected individuals is essential". For elderly HIV patients failing HIV treatment due to viral drug resistance, the newly developed CCR5 antagonists represent an attractive treatment option because they are active against drug-resistant HIV and because they have reduced potential for toxicity. Since HIV can develop resistance to CCR5 antagonists, we will identify the mechanism of drug resistance and potential strategies for its control, which will improve quality of life for patients with drug resistant HIV, especially for the elderly.